This invention relates to a process for the production of olefines such as ethylene, propylene and the like by circulating coke particles through both fluidized beds in a reactor and a heater, heating the coke particles by combustion of fuel and, if desired, a part of the coke particles in the heater and on the other hand, thermally cracking a heavy oil such as crude oil, its distillation residue, etc. with the heated coke particles as a heat carrier in the reactor (hereinafter referred to as "coke fluidized bed cracking method"), whereby a distillation residue obtained by the distillation of a liquid by-product in thermal cracking (hereinafter referred to as "by-product residue") can be used as a heat source for heating the coke particles and at the same time said by-product residue is converted into a low-calorie gas which has wide application and can be easily handled.
When olefines such as ethylene, propylene and the like are produced by thermal cracking of a petroleum fraction, light fractions having a high content of paraffinic hydrocarbon are suitable as feed stock, but such light fractions can be produced only in a low yield from a crude oil and are expensive. Therefore, it is very important that a heavy oil such as crude oil, its distillation residue and the like, which is relatively cheap and is readily available in secure, steady supply, is used as a starting material for producing olefines.
On thermal cracking of a heavy oil such as crude oil or its distillation residue, etc., under such a condition that ethylene can be produced in a high yield, a large amount of the liquid product is formed as shown in Table 1.
TABLE 1 ______________________________________ Product pattern obtained by thermal cracking of heavy oil (wt %) Middle East Atmospheric res- Vacuum residue Feed stock crude idue of Middle of Middle East Product oil East crude oil crude oil ______________________________________ Cracked gas 55 48 39 Liquid product By-product light oil 13 15 9 Middle fraction oil 13 10 6 By-product residue* 16 23 37 Coke 3 4 9 ______________________________________ *When middle fraction is not separated, the byproduct? residue is compose of middle fraction oil and the by? product residue described in Table 1.?
The liquid product can be separated by distillation into a by-product light oil (distilled oil) and the residue and, if desired, into three fractions consisting of by-product light oil, middle fraction oil and the residue.
By-product light oil is hydrocarbon oil containing a large amount of benzene, toluene, xylene and the like and useful as a raw material of benzene, toluene, xylene and the like. The residue, i.e. by-product residue, has generally a high content of sulfur and is a pitchy substance having a high softening point and thus its use is restricted.
Recently, it was proposed that the by-product residue can be used as a binder for blast furnace coke, but it has not come into wide use. Moreover, since the by-product residue has no solubility with the petroleum fraction of straight-run and has a high content of sulfur, its use as fuel in conventional boilers, etc., is difficult and it is necessary to develop special boilers in which the by-product residue is exclusively burned.
In general, when a heavy oil is thermally cracked, a large amount of coke is produced, adhered to the wall surface of the tube and prevents the heat transfer. Therefore, a heavy oil cracking operation can not be continued in a tubular thermal cracking furnace used for the thermal cracking of the light fraction. On the contrary, operational difficulties rarely occur and long run operation is possible in a fluidized bed cracker, because the resulting coke is deposited mainly on the surface of the particles forming the fluidized bed.
Coke fluidized bed cracking method is suitable especially for the thermal cracking of a heavy oil, since this method employs a fluidized bed system. That is, the operational difficulties caused by the deposition of the coke rarely occur and the by-product residue can be used as a heatsource for the thermal cracking.
In the coke fluidized bed cracking method, the heating of the coke particles in the heater is effected by combustion of the increment of the coke particles in combination with a heat exchange with the high-temperature, low-calorie gas fed from a combustion chamber arranged at the side of the heater. The main purpose of the former is to keep constant the amount of coke in the equipment by combustion of the increment of coke deposited on the surface of the coke particles by thermal cracking of feed stock, and thus only a small amount of heat is generated by such combustion. Namely, since the amount of coke deposited is small as shown in Table 1, the amount of heat generated by its combustion forms only a part of a required amount of heat (calorific value equivalent to about 20 wt % of the feed stock). Therefore, the major portion of the required amount of heat must be supplied by the high-temperature combustion gas from the combustion chamber.
Hitherto, as a heating method of the coke particles in the above case, it is known that the coke particles are heat exchanged with the high-temperature combustion gas generated by perfect combustion of fuel oil or fuel gas with air. On the other hand, the increment of the coke particles is burned with oxygen remaining in the combustion gas. However, this method has the following disadvantages:
(1) In order to control the amount for combustion of coke deposited on the surface of the coke particles, the proportion of oxygen remaining in the combustion gas fed from the combustion chamber must be controlled and therefore the combustion condition of coke can not be controlled irrespective of combustion condition in the combustion chamber.
(2) Since the percentage of excessive air in the combustion chamber is determined depending on the amount for combustion of coke as described above, its amount is generally about 0-50%, and the temperature in the combustion chamber becomes very high. Therefore, an expensive special material is required for fire brick of the combustion chamber.
(3) Concentration of nitrogen oxide in the combustion gas becomes higher, since the combustion chamber is maintained at a high temperature and under an oxidizing atmosphere.